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The WAD format was originally used in Doom and it acts similar to a ZIP file; it can contain many files as a single .WAD document. Unlike a ZIP file, there is no compression on the "files" inside a .WAD file, which are called lumps.

An important restriction of WAD files is that the lump names can be a maximum of eight characters long and when importing lumps into a .WAD, the filename extension is lost (this is because it is not needed - Doom is programmed to "know" the names of special lumps it needs to operate properly).